U.S. Pat. No. 4,555,938 describes a transom mounted marine speed sensor of the paddlewheel type, in which the paddles or blades are asymmetric in shape and formed of magnetized amorphous magnetic material. As the marine vessel passes through water, the paddlewheel rotates about an axis which is transverse the direction of travel. A Hall-effect device adjacent the paddlewheel senses the change in the magnetic field emanating from the paddles and generates an electrical signal directly proportional to the rotational speed of the paddlewheel. Generally, the rotational speed of the paddlewheel is linearly related to the vessel speed, but this is not always the case; especially at the extremes of high and low speeds.
The unenclosed design of the transom mounted paddlewheel sensor of the '938 patent has been found to improve linearity, at high speeds over that obtainable in enclosed transom mounted paddlewheel sensors and the alternate enclosed "through-hull" type paddlewheel speed sensors, i.e., sensors mounted through a hole formed in the vessel hull.
Various techniques have been devised, in an attempt to improve the high-speed performance of such enclosed paddlewheel speed sensors. Casani et al. in U.S. Pat. No. 3,531,988, suggest an open paddlewheel construction, in which four paddles are each supported by arms which define an open space between the paddle and the wheel shaft. Maeder et al. in U.S. Pat. No. 3,457,782, employs a waterwheel having a plurality of buckets, closed at their side, and recessed in the peripheral edge of the wheel. The slot within which the wheel rotates, is open on the downstream side. According to Maeder et al., this provides a relief space which prevents a buildup of pressure on that side, which would vary the rotational characteristics of the wheel.
Despite the above efforts, and that of other workers in the art, a need exists for a through-hull speed sensor with improved performance, especially at high speeds, which does not at the same time sacrifice low speed linearity and accuracy.